Cause of the Common-Mode Noise Suppression Effect of Y-Capacitor in Isolated DC/DC Converters

Electromagnetic compatibility (EMC) of power electronics is becoming increasingly challenging to manage. This is due, on the one hand, to the advancements in power electronics technology such as the spread of resonant converters, and on the other hand to the tightening EMC requirements. One of the most commonly used EMC measures in power electronics, and in particular isolated DC/DC converters, is adding a primary-secondary Y-capacitor to reduce common-mode (CM) electromagnetic interference (EMI). To provide a deeper understanding on how the Y-capacitor achieves this, CM impedance measurements on an isolated half-bridge LLC resonant DC/DC converter are presented in this paper. From the results, the reason behind the EMI mitigation effect of Y-capacitor can be observed which gives a more precise explanation on the underlying noise suppression mechanism and opens up an optimization opportunity for the technique.